The present invention concerns an ultrasonic transducer having a disk-shaped piezoelectric transducer element which is provided with a rotationally symmetrical, disk-shaped .lambda./4 matching element.
An ultrasonic transducer, of the type mentioned above, is described in German Patent Publication No. DE 39 11 047 ("the '047 publication"). In the transducer of the '047 publication, small changes in the diameter of the main surface of a matching element, relative to the diameter of the piezoceramic transducer element, influence oscillations of the transducer element to improve the efficiency and radiation characteristic of the ultrasonic transducer in conjunction with its small dimensions. The '047 publication also discusses that even small changes in the shape of the circumferential wall of the matching element can substantially change the oscillations of the transducer element. A straight line, which diverges from, or converges to, the piezoceramic element is specified as the configuration of the lateral line of the circumferential surface of the matching element. In this way, the diameter of the main surface of the matching element deviates slightly from the main surface of the piezoceramic transducer element. Depending on the thickness of the matching element relative to the diameter of the transducer element, slightly positively or slightly negatively curved lateral lines are also considered advantageous for the purpose of achieving a relatively centered high sound pressure.
However, the amplitude distribution occurring in the ultrasonic transducers discussed in the '047 specification has a relative minimum in the central region of the radiation surface. The amplitude rises in the radial direction, has its maximum at approximately half the radius, and drops off steeply towards the rim (i.e., the edge). This form of oscillation produces losses in the achievable sound pressure, and the shapes of sound lobes associated therewith have conspicuous side lobes. These losses can lead, in practical use, to faults and malfunctions.
Therefore, the object of the present invention is to provide an ultrasonic transducer of the type mentioned above in which, in conjunction with a compact design, a high sound pressure is achieved because of an improved form of oscillation with the lowest possible losses, and in which the suppression of side lobes is better than -30 dB.